PMF forming processes, particularly of cylindrical objects have hitherto been disclosed. See for example U.S. Pat. Nos. 3,654,787, 3,832,509, 4,143,532, 4,170,887, 4,531,393 and 4,442,846 and DE-4,436,615. In such PMF processes, a forming coil, particularly a single wind coil, is placed around the tubular workpiece to be formed. The passing of a short duration high intensity electric current through the coil, gives rise to eddy currents in the metal workpiece, this in turn creating a magnetic pressure, resulting from the repulsive forces between the forming coil and the metal workpiece. This magnetic pressure then brings to shrinkage of the tubular workpiece. The end result, whether there will be only forming, or whether the tubular workpiece will join or weld to another workpiece, depends on the exact circumstances of the process. For effective PMF forming high intensity magnetic fields are required and in order to achieve that, it is at times useful to use a field shaper, which is in fact a secondary coil, surrounded by several windings of a primary coil connected to an electrical energy source, typically a discharge capacitor.
An effective PMF forming process, requires that there be a close proximity between the forming coil (whether a primary coil connected to an energy source or a secondary coil--the field shaper) and the metal workpiece. This however may create technical difficulties in a number of cases. For example, in the case of joining of long steel pipes by a bridging sleeve, such as that described in U.S. Pat. No. 5,442,846, it is necessary to be able to open the coil to allow the sideways withdrawal of the pipes with the joining sleeve after joining (otherwise, it would have been necessary to extract the pipe by pulling it lengthwise, which is very often practically impossible).
Another example is the joining of cable lugs to an electric cable. Cable lugs are objects which have a tubular portion, adapted to receive an end of an electric cable and another portion, generally planar, having a hole which allows passage therethrough of a screw for tight attachment to another cable lug, to an electric power source or to a ground lead. In order to ensure tight contact between an electric cable and the cable lug, after insertion of an end of a cable into the tubular portion, the walls of this tubular portion are shrunk and tightly pressed onto the end portion of the cable contained therein. This has hitherto been achieved primarily by mechanical means.
The tight joining of a tubular portion of a cable lug and a cable end can also be achieved by means of PMF forming. However, the problem is that the planar portion of the lug is of a wider dimension than the diameter of the tubular portion and accordingly, if the forming coil would have been a standard fixed coil, it would have been practically impossible to extract the cable after forming. Apparatuses specifically designed for joining of a tubular workpiece to another tubular or cylindrical workpiece, are disclosed in U.S. Pat. Nos. 3,654,787, 4,531,393 and 5,442,846. However, the apparatuses disclosed in these U.S. patents are bulky and are not suitable for field use which is where the joining of the cable end and the cable lug takes place.
The forming apparatuses disclosed in these three patents, have all a common feature in that the two halves which are clamped together so as to define a PMF forming region around the workpiece to be formed, are each connected to a different pole of the electric discharge source. The problem is that the electric contact with the source has to be a very low resistance contact, and accordingly a bulky arrangement is required to ensure the maintenance of such a contact notwithstanding the fact that at least one of the two halves of the forming coil has to be displacable.